A stable voltage reference is required for a digital to analog converter (DAC) to output an accurate analog voltage. The accuracy of the analog voltage is particularly important for a DAC used in an ADSL line driver. The output power allowed for an ADSL system must stay within a tight range as defined by a standard (e.g. ITU 6.992.1 or 6.992.2). If the DAC in the ADSL line driver does not have a stable reference, the output power will not be well defined. The bandgap referencing technique has been widely employed for implementing a voltage reference source in bipolar integrated circuits, including circuits implemented in CMOS. FIG. 1 is a block diagram illustrating a conventional CMOS bandgap reference circuit. The area of transistor 102 is much larger than the area of transistor 104, usually by about a factor of 10. The emitters of the two transistors are connected to the noninverting and inverting inputs of op amp 106. The output of op amp 106 at node 110 is a reference voltage, V.sub.ref, that, ideally, is a stable bandgap reference.
In practice, V.sub.ref also depends on the offset voltage of op amp 106, V.sub.os, and the difference between the emitter-base voltages of transistors 102 and 104. V.sub.ref may be given by: ##EQU1##
The op amp offset is a significant error source because the offset is generally not proportional to absolute temperature (PTAT). The op amp offset error and other error sources are described in more detail in "A Precision Curvature-Compensated CMOS Bandgap Reference" by Bang-Sup Song and Paul R. Gray, IEEE J. Solid State Circuits vol. SC-18, no. 6, pp. 634-643, Dec. 1983, which is herein incorporated by reference for all purposes.
Eliminating the op amp voltage offset or reducing its effect could greatly improve the stability of the bandgap reference. Different approaches have been suggested for doing that. For example, one possible solution is to use chopper stabilization to null out the op amp offset voltage. However, this technique creates undesirable switching transients and the reference voltage is valid only during a portion of the clock period. That is not preferable for a DAC used in an ADSL driver and in other applications that require a continuous and stable reference.
Another approach has been suggested for reducing the relative effect of the offset voltage by increasing the relative contribution of the bipolar transistor base-emitter voltages. By using an area-ratioed stack of three closely matched bipolar transistors, it is possible to produce a basic reference voltage that is three times the silicon bandgap voltage. That reduces the effect of the offset factor by a factor of 3. The bandgap voltage is then given by: ##EQU2##
Yet another approach is to trim, as with a laser, component values of certain elements within the op amp in order to reduce or eliminate the op amp offset. However, this is costly. Extra steps are required during manufacturing and testing to perform the trim function.
Although these techniques reduce variation resulting from the offset voltage of the op amp, it would be useful if a more effective technique could be developed. Specifically, a technique for compensating for the op amp offset that provides a continuous reference; that does not require removing or switching the op amp out of the bandgap reference circuit; and that does not require additional steps during manufacturing and testing, is needed.